Vehicles:
Don't bother with planes for now - they arn't a threat to ground units as it currently stands unless you stand still and no one really does this. Their gun has a range of about 400M, rockets are more of a threat but eh, all vehicles can run easily.
Unlocking the CITV is a permanent upgrade which provides proximity scan and SOFLAM but it's real benefit is spawning on tanks. This makes it worth getting tank points asap as you can be a more mobile unit.
If you're going to go for tanks operate as a pair - the driver is responsible for pointing his tank in the right way (ALWAYS point your front armour at the enemy gun) and shielding the gunner as he repairs - whichever way you're looking at when you leave a tank is the direction you end up so to repair look behind you, get out, quick repair then get back in.
Don't bother with IR smoke - you can outrun all the things which are a genuine threat and the other options are better.
On Canals attack if you tank the IFV at spawn you
will die as you're so slow someone will javelin you from the shore - take the boats and setup mobile spawn points.
Against vehicles - stinger reload time exceeds the reload time of flares - unless you have a buddy with a stinger too just forget about them.
SOFLAM is great in theory - crap in execution. Javelin will not accurately hit targets in top attack profile mode if they are on the move as their steering is so poor. The only really decent use of SOFLAM is i) Scaring people, ii) guided shell tank upgrade.
On CQB maps with the tank take thermals - you'll need it to spot mines (which blow up with the main gun, sniper rifle and explosives only).
Mines stay around after death and through class changes - if you plan to be an assault engi on your first life take mines + explosives perk and lay out 6 in 3 groups of 2 (2 will kill or be close enough). Don't throw mines on top of each other, the engine wigs out and they move.
As a gunner on tank do not shoot randomly or at planes unless they are very close - the gun has a range of about 300M, gives away your position and will annoy your driver.
Infantry:
GOLDEN RULE OF FIGHTIN' - this game deliberately makes it so that you probably will lose most 1v2/1v3's unless you have all the surprise (you'll get suppressed, as long as one survives the other can spawn on him, they keep their position). This is because there are constraints on damage and on some guns - ammo. Rambo mode is seldom useful - do not be crap

Assault wins rush - a squad without one is not a squad you will change the tide of battle with. Put the medpacks down constantly whenever you stop and it's clear, you never know when you'll need than extra regen. You don't tend to need more than 1 great assault per squad - the other classes bring such large benefits (except maybe recon, their use isn't so great) that it's not worth 2.
Suppression is extremely underrated - It stops health regen, does brutal things to their visibility, allows for easy flanking. A suppression assist kill gives 50 suppression + 10 assist + <damage bonus> so you probably get more points for this than the actual kill. In general you want the support to be king here with the M249 (for RoF, aim) or the M240B (damage is higher). The other LMGs are OK but have tradeoffs that these 2 don't. Take extended mags, you want to be laying down the fire.
Supports hold points like no ones business - tripod is what you want here and a good place but move after a kill or two, don't sit waiting for mortar fire, move and be a credit to team.
C4 - 2 packs kills 99% of stuff - make sure you use your crate to keep yourself topped up. doesn't matter where you put it on the tank but I'd suggest the sides (front / back = run over if they have proximity).
Engis are slightly over powered currently with the SCAR-H as it's a 7.62 beast (kills in 1 less bullet than nearly every other gun) but the G36C has virtually zero recoil and is another great sub. Don't bother with the AA rocket, stick with the staples (RPG/SMAW or Javelin).
Javelin does 35 damage from
any direction (except maybe top attack mode? needs confirming) but the RPG damage is directional based. Shoot the back / sides always, the front gives terrible damage thats less than the jav (and you'll probably eat a shell),back does double this. Aim for the tracks not the turret in a side attack. In most maps you'll be wanting javelin for long range engagement (shooting stuff approaching shore etc...) and RPG for every other situation. The only exception to this is the javelin will lock onto the MAW scout without SOFLAM so if they are using one of these 300M in the air pop it down (it can't out run). Explosives perk will only give you 4 javelins which is a bad investment for their number (2 standard) and and damage - you'll need a support to fix this so consider if you'll actually get one before using it.
Recons get a bad rep because they snipe which is
almost without use in BF3 as you should be capping or pushing the waves. In 95% of situations a much better thing to do is grab a shotgun or PDW and run with the pack. Your mobile spawn point is amazing on conquest (but please hide it) and MAW on a rush with mortar / vehicle support is a combo the enemy will dread. If you snipe you're probably making your team lose, especially if you're on attack (in which case you'll probably notice the defense taunting you over chat) and a waste of tickets. Canals is particularly where recons can shine due to mobile respawns and the size / layout.
If you hear the ticking sound near a point it means someone is attacking - if you hear it after capping it means they are in capping range and you shouldn't leave - find him.
With all infantry remember there is a huge penalty for moving and shooting - so much so that crouch bursting is probably 2/3 times more accurate at >50M and you probably will hit twice in your initial burst, it's probably worth just bursting for this range.
Capping is greater in priority than any firefight in terms of usefulness, tickets and points.
Edit more stuff:
A single squad can influence a battle upto probably 32 player servers, after this you'll rely on your team mates a lot more. Don't lose hope, 100 tickets can be an easy turnaround on most maps if you get either a good lockdown or setup an armoured front of tanks.
Squad leaders - Q attacking points (you should be Q'ing every 3 seconds anyway) - it's more points, gives pub teams direction etc...
Q can be used (but it's poorly implemented) to tell others you need ammo / health. For some reason it's horribly delayed / ignores the obvious but you can help by just putting down a medpack/crate after each fight (IN COVER) anyway.
On er... (I only know maps by what they look like)... the desert map with the refinery, the first point of rush has a bajillion things that can be blown up - fuel tanks, tanks apon the refinery itself, blow it all up.
Without a doubt the best gun upgrade is IRNV but suppressor is core for me, you don't appear on the minimap and as I use 30 round/full auto guns I can kill 2 guaranteed - 3 with bursting without reload including the damage tradeoff (only really affects >30M fights). The only gun I don't take this on is LMG, I really want that instant suppression / initial damage combo here so they can't heal and they will try to seek cover making them food for explosives / team mates.
Don't knife, windup is far too long and the dogtags for medals are better, just headshot and be gone in a flash ready to kill the team mate who spawned on him 3 seconds ago.
Scopes greater than x4 do the SHINY LOOK AT ME thing, don't use them unless you like dying. Same with laser, it has some use but it will just get you killed to be honest.